1. Field of the Invention
The present invention relates to a technology for detecting a current position of a mobile object based on a series of frame images of a left side and a right side of the mobile object.
2. Description of the Related Art
Conventionally, a navigation device installed in a mobile object such as a vehicle provides a navigation service to a driver by measuring a position of the vehicle using a global positioning system (GPS). GPS performs positioning based on an arrival time of a radio wave transmitted from an artificial satellite. GPS has a positioning accuracy of several meters to several tens of meters. Further increase in the positioning accuracy can be expected in the future due to positioning using radio wave phase differences such as real time kinematic-GPS (RTK-GPS), and launchings of Galileo by the European Union (EU) and a quasi-zenith satellite by Japan. Therefore, GPS will remain to be a mainstream method of vehicle positioning.
The positioning accuracy of GPS is sometimes significantly reduced due to an influence of multipath when the vehicle is in a location in which the radio wave from the artificial satellite is unstable, for example, when the vehicle is under elevated structures or between buildings. To eliminate the influence of multipath, a following position detecting device is proposed. The position detecting device stores information on a position and a height of a structure such as a building. Then, the position detecting device detects the position of the vehicle based on the radio wave from a GPS satellite. The position detecting device judges whether a structure is present on a straight line connecting the GPS satellite and the vehicle. If the structure is present, the position detecting device corrects the position of the vehicle considering radio wave reflection of the structure (for example, Japanese Patent Laid-Open Publication No. 2005-195493).
In addition, in recent years, to accurately determine a current position of the vehicle, many navigation devices employ a method of adjusting, using map-matching, the current position of the vehicle in road data to a position that is close to an actual current position. More specifically, map-matching correlates the current position of the vehicle with a road link that is closest to a positioning coordinate determined by GPS, among road links connecting each node point on map data. In addition, map-matching corrects a position of the vehicle in a traveling direction by determining a travel distance of the vehicle based on a circumference of a tire and a speed pulse output in accordance with a rotation of the tire.
However, according to map-matching explained above, a function of enhancing the positioning accuracy in the traveling direction of the vehicle regarding where the vehicle is located on the road link (or on a road link array), is not provided. Therefore, GPS errors in the traveling direction of the vehicle are not resolved. Furthermore, errors in the traveling direction are increased depending on the map-matching algorithm.
When the position is corrected using the speed pulse, the circumference of the tire is affected by air pressure, tire friction, and the like. Furthermore, the current position of the vehicle is measured by an addition of traveled distances. Therefore, the errors accumulate as the travel distance of the vehicle increases.